Technical Article

The Filter in The Power Adapter Circuit

The three elements: the formation of a power adapter electromagnetic interference source of harassment,
transmission and disturbed equipment.Thus, the suppression of electromagnetic interference should
also be from the three aspects of manpower, to take appropriate measures.Should inhibit the source of harassment directly to eliminate the cause of
the disturbance; followed by the elimination of the coupling between the
disturbance source and the disturbed equipment and radiation, cutting electromagnetic
interference route of transmission; disturbed equipment immunity to reduce its
noisethe sensitivity.Interference suppression of
several measures are basically use to cut off sources of electromagnetic
disturbance and by coupling channel interference between devices.The commonly used method is the shielding, grounding and
filtering.

1) shielding technology can effectively suppress electromagnetic radiation
interference of switching power supply, good conductivity material to shield
the electric field, magnetic field shielding material with high permeability.Shield serves two
purposes, restrict internal radiation of electromagnetic energy leaking out of
prevent external radiation interference into the internal area.The principle is to use the shield body to electromagnetic
energy reflected, absorbed and guiding role.In order to suppress the switching power supply generated by radiation,
electromagnetic disturbance to other electronic devices, in full accordance
with the method of magnetic shielding to process shield, and then the entire
shield system enclosure and connected as one, will be able to electromagnetic
fieldseffective shielding.

2) the so-called
ground between two points and create a pathway to the electronic equipment or
components connected to certain known as "ground" reference point.The ground is an
important method of AC/DC Adapters device for electromagnetic
interference suppression, connected to the power supply certain parts of the
earth can play the role of interference suppression.Circuit system design should follow the principle of "ground",
if the formation of multi-point grounding, ground loops closed magnetic
induction noise will be generated when the magnetic field lines passing through
the loop.Nearest part is actually very difficult to achieve the "ground"
Therefore, in order to reduce the ground impedance to eliminate the impact of
the distributed capacitance taken by the planar-type multi-point ground, using
a conductive plane as the reference ground, to ground connected to the
reference earth.To further reduce the pressure
drop of ground loops, the the available bypass capacitor to reduce the
amplitude of the return current.In the
low-frequency and high-frequency circuit system coexist, the ground of the
low-frequency circuit, high-frequency circuit, a power circuit should be
connected separately, and then connected to a common reference point.

3) filtering is an effective
way to suppress conducted interference, electromagnetic compatibility design of
the equipment or system has an extremely important role.EMI filter as an important means to suppress power line conducted
interference can be suppressed against interference from the power grid to the
power supply itself, interference can be suppressed by the switching power
supply and feedback to the grid.In many dedicated
filter element in the filter circuit, further, such as the feedthrough
capacitor, three-terminal capacitors, ferrite magnet, they are able to improve
the filtering characteristics of the circuit.Appropriate design or
selection filter and properly install and use the filter, is an important part
of the anti-jamming technology.

When you select a filter, note the following:

(1) clear
operating frequency and to suppress interference frequency, such as the two are
very close, you need to apply a very steep filter frequency characteristics, in
order to separate the two frequencies;

(2) ensure that the filter can work reliably under high
pressure;

(3) continuous
pass filter to the maximum rated current, the temperature rise is lower, in
order to ensure the continuous operation of the rated current, does not destroy
the work performance of the filter element;

(4) frequency
characteristics of the filter to make it work with the design values ​​consistent with the
requirements specified value with it connected signal source impedance and load
impedance value equal to the design:

(5) The filter must have a shielding structure, shielding lid and the body
have a good electrical contact, the filter capacitor leads should be as short
as possible, the best selection of low-inductance of the short lead feedthrough
capacitors;

(6) have a high
reliability, protection for electromagnetic interference filter, and its
failure more often than the failure of other components are harder to find.

Install a filter should note the following:

(1) Power line
filter should be installed as close as possible to the local population from
the device power, do not let the power cord through the filter in the device
box circuitous;

(2) filter capacitor leads
should be as short as possible, so as to avoid lead inductance and capacitance
to resonate at lower frequencies;

(3) Filter the
grounding conductor through the large short-circuit current, will cause the
additional electromagnetic radiation, so the response to the filter element
itself is a good shielding and ground handling;

(4) the filter input and output lines can not cross, otherwise it will
cause crosstalk due to the filter's input and output capacitive coupling
passage, thereby reducing the filtering characteristics, the usual approach is
to add a separator between the input and the output terminal or shielding layer.

(1) Power line filter should be installed as close as possible to the local population from the device power, do not let the power cord through the filter in the device box circuitous;

(2) filter capacitor leads should be as short as possible, so as to avoid lead inductance and capacitance to resonate at lower frequencies;

(3) Filter the grounding conductor through the large short-circuit current, will cause the additional electromagnetic radiation, so the response to the filter element itself is a good shielding and ground handling;

(4) the filter input and output lines can not cross, otherwise it will cause crosstalk due to the filter's input and output capacitive coupling passage, thereby reducing the filtering characteristics, the usual approach is to add a separator between the input and the output terminal or shielding layer .